Wire loops serve to produce electrical connections between a semiconductor chip and a substrate. Depending on the application, different demands are placed on the wire loops. The wire loops are mainly produced by a so-called Ball-Wire Bonder. The Ball-Wire Bonder has a capillary with a longitudinal drill hole that opens out into a ring-shaped working surface. The capillary is clamped to the tip of a horn. The wire runs through the longitudinal drill hole of the capillary. The capillary serves to attach the wire to a connection point on the semiconductor chip and to a connection point on the substrate as well as to guide the wire between the two connection points. On producing the wire connection between the connection point on the semiconductor chip and the connection point on the substrate, the end of the wire protruding out of the capillary is first melted into a ball. The ball is then attached to the connection point on the semiconductor chip by means of pressure and ultrasound. In doing so, ultrasound from an ultrasonic transducer is applied to the horn. This process is called ball bonding. The wire is then pulled through to the required length, formed into a wire loop and thermocompression bonded to the connection point on the substrate. This last sub-process is called wedge bonding. After attaching the wire to the connection point on the substrate, the wire is torn off and the next bond cycle can begin.
From the U.S. Pat. No. 4,437,604, a method is known with which the wire loops are attached to both connection points by means of a wedge connection. This means that the formation of the wire ball is omitted and that both wire connections are produced by compressing the piece of wire under the capillary, i.e. the working surface of the capillary presses the wire against the connection point. Compressing the wire is done by applying a predefined force and ultrasound, generally with increased temperature. Because the wedge connections are produced with a Ball Wire Bonder with which the capillary presses the wire onto the respective connection point and not with a Wedge Wire Bonder with which a wedge bond tool serves to attach and guide the wire, the quality of the wedge connection on the first connection point generally does not fulfil the set requirements. FIG. 1 shows the first wedge connection produced on the first connection point 1. The wedge connection comprises a so-called wedge bond 2 and a so-called tail bond 3. In order to achieve good adhesion of the wedge bond on the first connection point, during bonding the wire 4 has to be pressed against the first connection point with a lot of energy, i.e. with a relatively high bond force and under the application of ultrasound. The junction between the wedge bond and the tail bond is therefore very thin or is even completely severed so that the wire is practically only connected to the connection point by the tail bond. Adhesion of the tail bond is naturally much weaker than the adhesion of the wedge bond as because of the geometry of the capillary on the one hand the connection area is very small and, on the other hand, the pressing force of the capillary and the effect of ultrasound here are very low. With the subsequent formation of the wire loop, considerable forces are exerted on the tail bond that lead to weakening of the tail bond and therefore to all the problems that are known in the trade as “wire sway”, “neck tilt”, “loop height” and “loop shape” variations or even lead to the tail bond being torn away from the surface. In principle therefore this method enables the production of a wedge-wedge connection using a Ball Wire Bonder. However, for the named reasons, the stability of the first contact and therefore of the wire loop is totally insufficient because of the naturally weaker tail bond. For this reason, the wedge-wedge method fell into oblivion and was only taken up again in recent years.
A method for producing a bump-wedge wedge wire loop between a semiconductor chip and a substrate is known from patent application US 2005-0054186. With this method, a bump is first applied to the connection point on the semiconductor chip and then the wire loop is produced in that the end of the wire protruding out of the capillary is attached to the bump as a wedge connection, the wire is pulled out to the required length and attached to the second connection point on the substrate as a wedge connection.
A method for producing a ball bump-wedge wire loop between a semiconductor chip and a substrate is known from patent application US 2004-0152292. With this method, a so-called bump is first applied to the connection point on the semiconductor chip and then the wire loop is produced in that the wire protruding out of the capillary is melted into a ball and attached to the substrate as a ball connection, the wire is pulled out to the required length and attached to the bump on the semiconductor chip as a wedge connection. This method is known as “Reverse Bonding”. It is not a wedge wedge method.
The two last mentioned methods enable the wire to be guided away practically parallel to the surface of the semiconductor chip. However “Reverse Bonding” has the disadvantage that a wire ball has to be formed for both wire connections which results in a comparatively long cycle time for the production of the wire loop. The other method has the advantage that only one wire ball has to be formed but has the disadvantage that the wire loop can often not be formed in the required shape.